Nucleating agents have been used for many purposes in silver halide photographic materials. For instance, hydrazines which have been most prevailingly used as nucleating agent have been adopted as development nucleus forming agents in direct positive silver halide emulsions of the internal latent-image type. In negative silver halide emulsions which form a latent image at the surface thereof, they have been used to increase sensitivity and/or contrast.
Silver halide emulsions which have sensitive nuclei predominantly inside the silver halide grains and which generally form latent images inside the grains, are called silver halide emulsions of the internal latent-image type. They are distinguished from silver halide emulsions of the kind which form latent images on the surface of the grains to an appreciable extent.
A method of forming direct positive images by subjecting silver halide photographic emulsions of the internal latent-image type to surface development in the presence of a nucleating agent, and the photographic emulsions or photosensitive materials to which such a method is applicable are known, e.g., in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552 and 3,317,322, British Patents 1,011,062, 1,151,363, 1,269,640 and 2,011,391, JP-B-43-29405, JP-B-49-38164 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-53-16623, JP-A-53-137133, JP-A-54-37732, JP-A-54-40629, JP-A-54-74536, JP-A-54-74729, JP-A-55-52055, JP-A-55-90940 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), and so on.
In the method described above for obtaining a direct positive image, it is more common to incorporate a nucleating agent in a photographic emulsion layer or another appropriate constituent layer of a photosensitive material than to add such an agent to a developer.
Hydrazine compounds are well-known as nucleating agents which can be incorporated in silver halide photosensitive materials.
In general, hydrazine nucleating agents can bring about a great difference between the maximum density (Dmax) and the minimum density (Dmin), so that they are superior in discrimination, However, they have the defect that they require high pH (i.e., pH&gt;12) for processing.
Heterocyclic quaternary ammonium salts are known as the nucleating agents which can function in processing at a low pH ( pH.ltoreq.12 ) . Examples of such salts are disclosed in U. S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Patent 1,283,835, JP-A-52-3426, and JP-A-52-69613. In particular, propargyl- or butinyl-substituted heterocyclic quaternary ammonium salt compounds disclosed in U.S. Pat. No. 4,115,122 function as nucleating agents which are excellent in discrimination when incorporated in direct positive silver halide emulsions. Other alkynyl-substituted heterocyclic quaternary ammonium salt compounds similar in function to the above-cited examples are disclosed, e.g., in U.S. Pat. Nos. 4,471,044, 4,828,973 and 4,859,579, JP-A-62-210451, JP-A-63-121042, JP-A-01-191132, and U.S. Pat. No. 4,306,016.
In addition, dihydropyridine compounds are known as highly active nucleating agent with an aptitude for low pH processing. Examples thereof are disclosed, e.g., in U.S. Pat. No. 3,719,494, JP-A-62-291637, and British Patent 2,172,710B. However, those compounds do not exhibit satisfactory stability with the lapse of time.
On the other hand, quaternary ammonium salt type compounds are known to function as development accelerators in silver halide negative emulsion systems. Examples of such compounds are disclosed in U.S. Pat. No. 4,135,931, JP-A-52-114328, JP-A-52-121321, German Patent 2,647,940 and Belgian Patent 721,568.